How helpful are surveillance cameras?

After the Boston Marathon bombings, Mayor Emanuel extolled the value of video surveillance cameras and promised to keep adding them in Chicago, which already has the nation's most extensive network. In Boston, the cameras didn't deter the killers, but they did provide footage that identified the two men believed to have carried out the attack. Clearly, they can be a vital tool in solving crimes.

So I asked the city for data on how often that happens. The city now has 22,000 cameras in its network, and a spokesman told me that since 2006, they have helped solve 4,500 crimes.

That may sound like a lot, but it's not -- not when you consider that in 2010, more than 150,000 serious crimes were reported. Over the past six years, there have been more than a million such incidents. Surveillance cameras have helped solve less than one-half of 1 percent of them.

Another way to look at their performance is how many crimes each camera solved. It works out to one for every five cameras. The average camera never solves a crime.

That suggests a couple of things. One is that we probably have too many cameras already. Another is that any additional ones will be even less effective -- since the highest-crime areas presumably have already been covered.

The mayor thinks the value of these devices is obvious, and it is -- as long as you don't look at the data. Then you have to wonder if the money spent on them couldn't be spent in a more effective way.